In general, a bicycle is designed to move forward by driving the wheels with the force from two legs in a state where a rider is seated on a saddle. The pedals are alternately stepped down with two feet with a rider on the saddle holding a handle to rotate the wheels connected through a chain.
The wheels in general are provided in pair. In rare case, there may be only one wheel or three wheels. Such a bicycle has been widely used as one of major transportation means which are driven by manpower. In recent years, the bicycle is being applied for various purposes as a recreation means while providing a health promotion effect as an exercise tool, and needs for the same increases thanks to its environment friendly characteristics.
As illustrated in FIG. 1, the bicycle is configured in such a way that a plurality of frames 10a to 10g are connected in a trust structure. There are provided sprocket wheels 17a and 17b, a chain 19, etc. which are able to transfer a driving force which generates by front and rear wheels 12a and 12b, a handle bar 14, a saddle 22, pedals 18 and an arm 16, to a rear wheel 12b, the operation of which is based on the supporters of the above-mentioned frames 10a to 10g. 
At this time, the height of the saddle 22 can be adjusted through a saddle supporter 20 on the top of a saddle frame 10a supported by a main shaft frame 10b and a support frame 10f which are among the plurality of the frames 10a to 10g. 
As illustrated in FIG. 2, the height adjustment of the conventional saddle 22 may be obtained by an engaging force of a fixing screw 24 provided at a side portion of the saddle frame 10a in a state where the bottom of the saddle supporter 20 is inserted into the top of the pipe-shaped saddle frame 10a. 
In addition, a support 26 is fixed at the top of the saddle supporter 20 by a conventional method, for example, a welding method, etc., and a saddle plate 28 forming a frame of the saddle 22 is installed on the support plate 26.
As seen in the installation structure of the conventional saddle 22, a height adjustment in general is performed based on the engaging force of the fixing screw 24 with respect to the saddle frame 10a. The upward, downward direction angles of the saddle corresponds to a user's body type and a user's posture and habit when riding, and its adjustment is necessary, but there is a hard relationship for the adjustment.
In addition, many persons, for example, a family or colleagues, can use a bicycle together. At this time, a lot of time may be necessary for each person to adjust the angle of the saddle 22 based on his body type, thus causing a lot of problems.
Furthermore, when riding a bicycle, since the angle of the saddle 22 cannot cope with any change in the user's posture when riding on a flat ground and a sloped road, for example, an uphill or a downhill, the user may have many inconveniences.
For example, when the user rides a bicycle on a uphill or a downhill, the user's posture tends to incline in a forward or backward direction of the bicycle. In this case, since the angle of the saddle 22 is fixed, the user may feel any inconvenience in his posture, thus causing a high risk to accident.